Voltage reference circuits are required in a wide variety of electronic circuits to provide a reliable voltage value. In particular, such circuits are often designed to ensure that the reliable voltage value is made substantially independent of any temperature variations within the electronic circuit or temperature variation effects on components within the electronic circuit. Notably, the temperature stability of the voltage reference is therefore a key factor. This is particularly critical in some electronic circuits, for example for future communication products and technologies such as system-on-chip technologies, where accuracy of all data acquisition functions is required.
In the field of the present invention, a bandgap voltage reference is known to produce an output voltage very close to a semiconductor bandgap voltage. For Silicon, this value is about 1.2V. Thus, a sub-bandgap voltage is understood to be below 1.2V for Silicon.
Generally, there are two known basic components that are used to generate a bandgap voltage reference output. A first component of such electronic circuits is usually a directly-biased diode, for example a base-emitter voltage of a bi-polar junction transistor (BJT) device, with a negative temperature coefficient. A second component of such electronic circuits is a voltage difference of directly biased diodes that is configured as providing an output proportional to absolute temperature voltage. Thus, by arranging the outputs of these components in an appropriate ratio, the sum of the outputs is able to provide a voltage reference that is almost independent of temperature. Notably, in current electronic circuits, the output voltage of a bandgap voltage reference under such conditions is approximately 1.2V.
Unfortunately, the base-emitter voltage of a bipolar transistor does not change linearly with transistor temperature. Hence, it is known that a simple bandgap circuit that sums only two components in the above manner has an output parabolic curvature response and a second-order temperature dependence. Therefore, in order to increase the temperature stability of the voltage reference, a second-order compensation circuit is generally applied.
The temperature dependence of a voltage reference can be seen in the temperature dependence of the base-emitter voltage of a forward-biased bipolar transistor, as illustrated in equation [1]:
                              Vbe          =                                    Vg              ⁢                                                          ⁢              0                        -                                          (                                                      Vg                    ⁢                                                                                  ⁢                    0                                    -                                      Vbe                    R                                                  )                            ⁢                              T                                  T                  R                                                      -                                          (                                  n                  -                  x                                )                            ·                                                k                  ·                  T                                q                            ·                              ln                ⁡                                  (                                      T                                          T                      R                                                        )                                                                    ,                            (        1        )            where:
Vgo: is the bandgap voltage of silicon, extrapolated to ‘0’ degrees Kelvin,
VbeR is the base-emitter voltage at temperature Tr,
T: is the operation temperature,
TR: is a reference temperature,
n: is a process dependent, but temperature
independent, parameter,
x: is equal to 1 if the bias current is PTAT and goes to ‘0’ when the current is temperature-independent, i.e. if a current, flowing through a diode is not temperature-dependent, then Vbe changes in accordance with its own temperature parameters. In a case where a current flowing through a diode is temperature-dependent, then Vbe changes in accordance with its own and current temperature parameters. Thus, x=1 if a bias current is linearly proportional to temperature, and x=0,if it is temperature independent.
k: is Boltzmann's constant, and
q: is the electrical charge of an electron.
It can be seen, that the first term in [1] is a constant, the second term is a linear function of temperature, and the last term is a non-linear function. In first order bandgap reference circuits, only the linear (second) term from [1] is usually compensated. The non-linear term from [1] stays uncompensated, thereby producing the output parabolic curvature.
FIG. 1 illustrates a schematic diagram 100 of a conventional first order bandgap reference circuit, where the output voltage Vref 125 is assumed to have exact first order temperature compensation. The circuit comprises of positive and negative temperature dependant current generators, based on Q1 120, Q2 122, m4 124, r1 126 and current mirrors 110, 112. The circuit further comprises an output stage 130, which is based on resistor r2 and Q3 as a diode. Q1 120 produces a negative temperature-dependant current. The Vbe difference between Q1 120 and Q2 122 is applied to resistor r1 126. As a result the Q2 emitter current is proportional to delta Vbe, divided by r1 126, and has positive temperature-dependence.
Current mirror m1 110, m2 112 and transistors Q1 120, Q2 122 and m4 124 produce negative feedback to compensate for the collector current of Q1 120 and the drain current of m1 110. Current mirror m2 112 and m3 114 produce an m3 drain current proportional to the collector current of Q2 122. Transistor m4 124 and current mirror m5 116 and m6 118 form an m6 drain current that is proportional to the base currents of Q1 120 and Q2 122. Both drain currents of m3 114 and m6 118 flow through the output stage, thereby producing a voltage drop on diode Q3 with negative temperature-dependence and a resistor r2 with positive temperature-dependence. In a case where their temperature coefficients are equal to each other, then the output voltage (125) will be temperature compensated.
The exact first order temperature compensation is expressed by:
                                          V            refBG                    =                                    Vg              ⁢                                                          ⁢              0                        -                                          (                                  n                  -                  x                                )                            ·                                                k                  ·                  T                                q                            ·                              ln                ⁡                                  (                                      T                                          T                      R                                                        )                                                                    ,                            (        2        )            where:
VrefBG: is an output voltage of the bandgap reference.
Hence, the output voltage 125 of a conventional bandgap reference is around Vgo, which is approx. 1.2V with several millivolts (mV) of parabolic curvature caused by the non-linear term from [2].
However, the trend in high performance electrical equipment, particularly portable communication equipment, is that a supply voltage of 1.5V or less needs to be used. Thus, in the context of the present invention, with battery-powered portable equipment such as an audio player or a camera, 1.5V is an initial voltage for battery voltage source, for example an ‘A’-size. If a battery is ‘discharged’ then the voltage falls below 1V.
U.S. Pat. No. 6,157,245,describes a circuit that uses the generation of three currents with different temperature dependencies together and employs a method of exact curvature compensation. A significant disadvantage of the circuit proposed in U.S. Pat. No. 6,157,245 is that it proposes five ‘critically-matched’ kohm resistors −22.35, 244.0, 319.08, 937.1 and 99.9. The large resistance ratio (up to 1:42) and the large spread of the ratios (from 1:4.5 up to 1:42) will be problematic and excessive mismatching of the resistors would be expected.
Furthermore, the trimming procedure to attempt to accurately and critically match the five resistors becomes too expensive for the circuit to be used in practice. Therefore, such a circuit is highly impractical for mass-produced devices.
The paper by P. Malcovati et al, titled “Curvature-Compensated BiCMOS Bandgap with 1-V Supply Voltage”, published in the IEEE Journal of Solid-State Circuits, vol. 36, No. 7, July 2001, pp. 1076-1081, also proposes a complicated circuit that includes an operational amplifier, five critically-matched resistors as well as three critically matched bipolar transistor groups.
Thus, there exists a need in the field of the present invention for a sub-bandgap voltage reference that is able to generate a fraction of 1.2V, notably with temperature stability comparable to current sub-bandgap voltage references.